The invention described herein relates to grids for nuclear reactor fuel assemblies and more particularly to an improved grid structure which helps assure precise attachment and alignment of control rod guide thimble sleeves in grid straps while still making springs and dimples accessible excessable for adjustment.
Conventional designs of grids for nuclear reactor fuel assemblies include a multiplicity of interleaved straps of egg crate configuration designed to form cells which accept fuel rods and control rod guide thimbles. A peripheral strap having the same width as the interleaved straps encloses the interleaved straps to impart strength and rigidity to the fuel assembly. In order to minimize the lateral displacement of fuel rods and to improve the flow characteristics of an assembly, a number of grids are spaced along the fuel assembly length. These grids are held in place by brazing or otherwise affixing control rod guide thimble sleeves to the grid, and guide thimbles are then axially inserted therethrough and subsequently secured to the sleeves. Each sleeve is short and projects only a few inches beyond the grid upper and lower surfaces. Control rod guide thimbles extend through the axially aligned sleeves in each of the grids along the fuel assembly length, and are secured thereto by bulging the guide thimble walls outwardly at a point immediately above and below the guide thimble sleeves. The control rods then axially reciprocate in the sleeves during reactor operation. When assembled, the fuel assembly includes great strength and rigidity and the fuel rods are effectively held against lateral displacement because of the cooperative effect provided by the grids.
Different methods are utilized for attaching the guide thimble sleeves to the straps in the grids. In one well known design, the sleeve is brazed directly to the exposed surface of the interleaved straps which comprise the grid. In still another design, the sleeve is provided with openings such that the sleeve is permitted to straddle the straps but nevertheless effectively anchor it in position. The disadvantages inherent in these designs are that the sleeves may not be precisely aligned so that the axes of a number of sleeves in vertically aligned grids are not the same, and as a result, the center line of control rod guide thimbles which are secured in the sleeves may not fall on the sleeve axes. As a result binding of control rods which are adapted to move vertically in the guide thimbles during reactor operation, may occur in the guide thimbles during operation. Also, there is no convenient way to adjust the springs and dimples within grid cells immediately adjacent to the cell which contains the control rod guide thimble. Further, known designs do not provide accurate methods for locating sleeves in the grid to achieve true alignment with sleeves in other grids.